1. Field of the Invention
The present invention relates to semiconductor memory device and, more particularly, to a semiconductor memory device comprising a variable resistance element which varies in resistance based on stored data.
2. Description of the Related Art
A magnetic random access memory (MRAM) is known as a kind of variable resistance memory. An MRAM is a device which performs memory operations by storing “1” or “0” data in a memory cell using the magnetoresistive effect, and is considered a candidate for a universal memory device characterized by non-volatility, high integration, high reliability, low power consumption, and high-speed operation.
There have been reported a great number of MRAMs using elements that show the tunneling magnetoresistive (TMR) effect among the magnetoresistive effects. A commonly used TMR effect element is a magnetic tunnel junction (MTJ) element which has a stacked structure including two ferromagnetic layers and a nonmagnetic later interposed therebetween, and uses change in magnetoresistance caused by a spin-polarized current. An MTJ element can assume either a low-resistance state or a high-resistance state according to the magnetization alignment of the two ferromagnetic layers. The low-resistance state is defined as “0”, and the high-resistance state is defined as “1”.
Data is read from an MRAM using an MTJ element by a method of determining a resistance of the MTJ element by applying a predetermined reading voltage to the MTJ element, and comparing an output signal current with a reference current. Alternatively, the data reading is performed by a method of determining a resistance of the MTJ element by applying a predetermined reading voltage to the MTJ element and comparing an output signal voltage with a reference voltage.
U.S. Pat. No. 6,385,109 discloses a method of generating a reference voltage at which an intermediate current between a “0” memory cell current and a “1” memory cell current upon application of a certain reading voltage is controlled to flow through a reference cell. However, since the method disclosed in the above-described document requires many operational amplifiers for generation of the reference voltage, the problems of increase in power consumption and circuit area are caused.